The present invention relates to a torque or angle of torsion measuring device with at least one pair of screening elements facing each other and concentrically arranged about a torque transmitting shaft, each element comprising a plurality of conductive zones, one of the elements of said pair being coupled to an end of the shaft, the other element being coupled to the other end of said shaft and at least two coils concentric to the shaft being provided in the vicinity of said elements and separated by these elements.
Such a device for converting an angle of torsion of a shaft into a digital code is known from the publication "Measurement Technique", Vol. 21, No. 7, July 1978, New York (U.S.), p. 930-934. However, the screening elements of the pair are provided with a different number of conductive zones and the device operates in accordance with the principle of the displacement of moire fringes for producing a digital output signal.
The patent GB No. 2 097 131 discloses a torque transducer comprising two pairs of wings, the wings of each pair facing each other and having a determined relative angular shift. The two pairs of wings permit a differential measurement. To each pair of wings is associated a coil in intimate contact with one of these wings. However, the relative angular shift of the wings of a pair causes a variation of the impedance of the associated coil such that in spite of the differential effect, the precision of the measurement is limited.
Further, there is known from the U.S. Pat. No. 4,356,732 a device in which the zones of the two elements are in alignment for a given initial torque. The coils are supplied with a high frequency signal and they induce eddy currents in the conductive zones of the elements. The energy which is necessary for creating these eddy currents is delivered by the coils which in turn produces a change of the impedance of these coils. The establishment of the eddy currents is in turn influenced by the relative angular shift of the elements when a torque is applied to the shaft, that is by the opening or closing of the non conductive regions between the conductive zones of the elements. Thus, the measurement of the variation of impedance of the coils permits to determine the relative angular shift of the elements and thus the torque applied to the shaft. However, like in the preceding case, in such a device, the precision of the measurement is limited because of the relatively weak variation of impedance of the coils, even for an angle of torsion as high as e.g. 20.degree. C. Further, the coils are sensitive to the variations of temperature so that the variation of impedance caused by the relative angular shift of the elements depends also on the temperature.